Background: Transcription elongation is one step in the multistage process of transcription that involves the polymerization of nucleoside monophosphates into a ribonucleotide transcript catalyzed by the multisubunit complex RNA Pol II. Transcriptional regulation involves positive factors that promote sufficient elongation rates and processivity as well as negative elongation factors that repress transcription elongation. SUPT5H is a subunit of the DRB sensitivity inducing factor (DSIF) along with SUPT4H that functions as a negative elongation factor. DSIF associates with RNA Pol II and may inhibit elongation by pausing and terminating RNA Pol II activity.